Fillers are typically included in the rubber compositions utilized in manufacturing tires, hoses, belts, windshield wiper blades, floor mats, and other rubber products. The fillers are chosen to enhance certain physical characteristics of the rubber composition. The addition of fillers tends to improve the physical properties of the polymer matrices to which they are added. For instance, the inclusion of carbon black and silica in tire tread rubber compounds is critical to attain required strength and tread wear characteristics. However, tires that are made with rubber compositions that contain high levels of fillers typically exhibit a higher level of hysteresis which is a disadvantage because it results in the tire having more high rolling resistance which in turn leads to a higher level of fuel consumption. The continuing desire for improved vehicle safety and low fuel consumption translates into a demand for tire rubber compounds that provide high strength, good tread wear characteristics, good traction, and low rolling resistance. For instance, tire treads are expected to have high abrasion resistance in order to provide a long life expectancy, to exhibit good anti-skid properties in both wet and dry conditions, and to have low hysteresis characteristics at elevated temperatures in order to provide low rolling resistance and reduced dynamic heat build-up.
The utilization of silica in tire tread rubber can lead to lower rolling resistance and better traction on ice, snow, and wet road surfaces conditions when compared with carbon black-based tire rubber compositions. However, one important feature of silica based tire tread compounds is that they typically rely on the utilization of solution styrene-butadiene rubber (SSBR) as the major component of the elastomer system. This is because the use of silica fillers in combination with emulsion styrene-butadiene rubber (ESBR) typically results in less desirable compound performance. Another negative aspect of using silica as a filler in rubber compounds is that it typically requires a relatively high level of an expensive silane coupling agents to attain the required level of interaction with the rubber in the composition. Still another drawback associated with using silica as a filler is that it requires better mixing temperature control than is normally needed in mixing rubber compounds. For these reasons the inclusion of silica as a filler in rubber compounds generally increases the cost of the compound appreciably.
U.S. Pat. No. 5,227,425 discloses a sulfur-vulcanizable rubber composition obtained by thermomechanical working of a conjugated diene compound and an aromatic vinyl compound prepared by solution polymerization in a hydrocarbon solvent having a total content of aromatic vinyl compound of between 5% and 50% and a glass transition temperature (Tg) of between 0° C. and −80° C. with 30 to 150 parts by weight per 100 parts by weight of elastomer of a silica having a BET surface area of between 100 and 250 m2/g, a CTAB surface area of between 100 and 250 m2/g, an oil absorption measured in DBP of between 150 and 250 ml/100 g, and an average projected area of the aggregates greater than 8500 nm2 before use and between 7000 and 8400 nm2 after thermomechanical mixing as well as the additives conventionally employed, with the exception of the sulfur vulcanization system, comprising at least one heat step reaching a temperature of between 130° C. and 180° C. for a suitable period of time of between 10 seconds and 20 minutes which is a function of the temperature selected in order to carry out the mechanical work and of the nature and volume of the components subjected to the mechanical work, followed by a finishing step consisting of the incorporating of the vulcanization system by mechanical work at a temperature below the vulcanization temperature.
U.S. Pat. No. 5,227,425 illustrates the need for using expensive synthetic rubbers made by solution polymerization in rubber compounds that are silica filled. For instance, expensive solution SBR is typically used in conjunction with silica and a silane coupling agent to achieve good tire rolling resistance and wet traction performance. This approach to using silica fillers in solution SBR also requires good control in compound mixing.
U.S. Pat. No. 5,395,891 and U.S. Pat. No. 6,127,488 to Obrecht disclose the use of crosslinked polybutadiene and crosslinked styrene-butadiene copolymers, respectively, as fillers for rubber compounds. The purported benefit of the inclusion of these crosslinked rubber particles in rubber formulations is lower hysteresis. Additionally, polybutadiene gels have been shown to impart improved abrasion resistance while styrene-butadiene copolymer gels have demonstrated improved tire wet traction characteristics. U.S. Pat. No. 6,133,364, U.S. Pat. No. 6,207,757, and U.S. Pat. No. 6,242,534 to Obrecht et al disclose a rubber composition comprising a crosslinked elastomer particle and a non-crosslinked elastomer. The crosslinked elastomeric particles are functionalized to impart unique surface reactivity for improved coupling with the non-crosslinked host elastomer as well as the subsequent ability to employ coupling agents other than the conventional sulfur-bridged bis-alkoxy silanes. The benefits of these gels in tire tread compositions are reported to be lower hysteresis, improved abrasion resistance, and improved wet traction. However, using these gel particles as fillers shows some critical drawbacks such as inferior mechanical properties (e.g. 300% moduli, tear and abrasion resistance). U.S. Pat. No. 6,620,866 discloses a rubber mixture containing agglomerated rubber gels. The purpose of using the high-pressure agglomerated gels is to improve the recognized deficiency in the mechanical properties.
U.S. Pat. No. 6,747,095 and U.S. Pat. No. 6,653,404 to Konno et al disclose a rubber composition comprising as essential components a crosslinked polymer particle and a non-crosslinked rubber component. The rubber composition may be used to obtain a vulcanized rubber. The crosslinked polymer particles are intended for use as filler to provide a rubber composition having good processability and handling performance as well as improved tensile strength and wear resistance when vulcanized.